Fix a minor bug in blocking FIFO queue.
Fix a minor bug in blocking FIFO queue.

We block a thread only when the queue is full, and there are still
entries left for adding the queue.

silky/FlashGraph,icoming/FlashGraph,icoming/FlashGraph,icoming/FlashGraph,icoming/FlashX,zheng-da/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashGraph,flashxio/FlashX,zheng-da/FlashX,flashxio/FlashX,zheng-da/FlashX,zheng-da/FlashX,flashxio/FlashX,zheng-da/FlashX,flashxio/FlashX,silky/FlashGraph,silky/FlashGraph,icoming/FlashX,silky/FlashGraph,icoming/FlashX,flashxio/FlashX,flashxio/FlashX,silky/FlashGraph,icoming/FlashX